Light source, and a field emission cathode

ABSTRACT

The light source, comprises an evacuated container having walls, including an outer glass layer ( 23 ) which on at least part thereof is coated on the inside with a layer of phosphor ( 24 ) forming a luminescent layer and a conductive layer ( 25 ) forming an anode. The phosphor ( 24 ) is excited to luminescence by electron bombardment from a field emission cathode ( 40 ) located in the interior of the container. The field emission cathode ( 40 ) comprises a carrier having a diameter in the mm range. At least a portion of the surface of the carrier is provided with a conductive layer having surface irregularities in the form of tips, having a radial extension being less than about 10 μm. Due to the geometry and the tips, the electric field is concentrated and amplified at the field emission surface.

FIELD OF THE INVENTION

The present invention relates to a light source according to theintroductory portion of claim 1, especially a light source forillumination. Further, the present invention relates to a field emissioncathode according to the introductory portion of claim 21.

BACKGROUND OF THE INVENTION

One common type of light sources is the fluorescent tube. It has manyadvantages, but suffers from serious drawbacks. For example, there isalways a delay after the power has been turned on until it starts tooperate giving full light. It needs complicated control equipment, whichrequires space. To obtain light with a source of this kind it isunfortunately necessary to use materials having negative environmentaleffects. It is for example a big disadvantage that mercury has to beused in this type of light sources.

Cathodolumninescent light sources is another interesting type of lightsources. Such light sources, including an evacuated envelope containinga grid and a heated cathode, for emission of electrons, are known fromGB, A, 2 070 849 (The General Electric Company Limited), GB, A, 2 097181 (The General Electric Company PLC), GB, A, 2 126 006 (The GeneralElectric Company plc) and GB, A, 2 089 561 (The General Electric CompanyLimited). Be insides of the envelopes are covered with a layer ofphosphor of an electron-responsive type. These cathodoluminescent lampshave essentially the form of an electric bulb.

Since these light sources all have a heated cathode, the cathode has tobe heated by special means, before the emission of light starts.

The use of electrons exciting phosphor to luminescence has the effectthat more heat is produced than in comparable fluorescent tubes. It istherefore advantageous if the active surface, for the emission of lightand for the necessary heat dissipation, is large. The cathodoluminescentlamps shown in the documents mentioned do not have optimal surfaces. Toovercome the drawbacks and problems with the fluorescent tubes andcathodoluminescent light sources, light sources having field emissioncathodes were developed.

A light source of this kind is disclosed in U.S. Pat. No. 5,588,893(Kentucky Research and Investment Company Limited). A field emissioncathode is arranged inside an evacuated glass container having aluminescent layer arranged on its inner surface. A modulator orextraction electrode is provided between the cathode and the luminescentlayer. The cathode includes carbon fibers, arranged in bundles,preferably in a matrix, on a substrate. The content of U.S. Pat. No.5,588,893 is incorporated herein by reference.

However in the last-mentioned known light source, electrons are emittedonly in a direction perpendicular to the substrate. Also, there is noindication in the document how to produce the light source in acost-efficient way.

The above mentioned U.S. Pat. No. 5,588,893 (Kentucky Research andInvestment Company Limited) also discloses a field emission cathode ofthe kind mentioned above. The cathode disclosed includes carbon fibres,arranged in bundles, preferably in a matrix, on a substrate. Thedocument also discloses a method including treatment of the emittingsurfaces in order to achieve a cathode with higher efficiency thanprevious cathodes.

Further, WO, A1, 98/57344 (LightLab AB) and WO, A1, 98/57345 (LightLabAB) disclose light sources having cylindrical geometry and employingfield emission. In order to obtain the necessary electric field forfield emission, the mentioned light sources include grids or modulatorelectrodes arranged close to the field emitting surfaces of thecathodes. In those light sources a relatively high electric field has tobe created between the cathode and the grid, and the distance betweenthe field emitting surfaces and the grid has to be small and uniform inorder to achieve a sufficient electric field for field emission and gooddistribution of electrons emitted from the cathode.

A further document, WO, A1, 97/07531 (Silzars et. al.) discloses alighting apparatus including a field emission cathode. The cathode isbuilt up of one or more fibers. The fibers are very thin, having adiameter less than 100 microns, and preferably less than 10 microns. Thediameters are selected in order to achieve field emission at reasonablevoltages. A construction according to this document having one fiberwill be inoperative if the fiber is broken. Since the fiber is verythin, the probability of that it breaks appears to be high. However, theprobability is probably somewhat lowered by arranging more than onefiber in parallel, for redundancy. Moreover, the electron emissionsurface is very small due to the small diameter of the fiber(s).

Previously known field emission cathodes are often of a complicated andfragile construction, especially as concerns the mountings and theattachment of field emitting bodies.

It has been found in connection with cathodes including standard carbonfibers and a grid that the electrical fields acting between the cathodeand a grid or an anode can cause individual fibers to get loose fromtheir carrier if they are not safely secured thereto. Once loose, thefibers will, in most cases, be attracted by the grid and cause a shortcircuit between the cathode and the grid, until it burns off after sometime due to the resulting current through the fibres.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a light source and a fieldemission cathode, respectively, providing a concentrated electric fieldat the field emission surface(s), and by which at least some of thedrawbacks above are eliminated or reduced.

These and other objects are attained by the features set forth in theappended independent claims.

By the features in claims 1, 12 and 34, 47, it is achieved a lightsource, a field emission cathode and an alternative light source andfield emission cathode, respectively, having a long life, with highefficiency and stability, which can be produced at low cost.

By the features in claims 1, 12 and 34, 47, it is achieved a lightsource, a field emission cathode and an alternative light source andfield emission cathode, respectively, having a sufficient electric fieldfor field emission with good distribution and high emission of electronsfrom the cathode.

By the features in claims 1, 12 and 34, 47, it is achieved a lightsource, a field emission cathode and an alternative light source andfield emission cathode, respectively, in which field emission can beobtained without the use of a grid or extraction electrode.

By the features in claim 1, further, a light source without a startingup period is achieved, i.e. when the power is turned on, the lightstarts immediately, thanks to the use of a field emission cathode. Alight source with no need for materials having negative environmentaleffects is also achieved.

By the features in claims 1 and 34, further, a light source having afield emitting cathode of simple and robust construction is obtained.

By the features in claim 5, further, a light source having a largeactive light emitting surface is achieved. This efficient use of thesurface renders it possible to achieve a light source having a highlight emission in relation to the heat produced.

By the features in claims 21 and 47, further, a field emitting cathodeof simple and robust construction is obtained.

By the features in claims 21 through 33 and claims 47 through 54 a fieldemitting cathode is obtained which further provides for a high emissionand uniform distribution of emitted electrons, in particular through acylindrical surface region surrounding the cathode. A cathode with lowinterference between the field emitting surfaces is also achieved.

Further features and advantages will be apparent from the dependentclaims and the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a longitudinal section of an embodiment of alight source according to the present invention,

FIG. 2 shows schematically a cross section taken at II—II in FIG. 1,

FIG. 3 shows schematically the cathode and the anode of FIG. 2, and

FIG. 4 shows schematically a cross section of an alternative embodimentof a light source according to the present invention.

FIG. 5 shows schematically a cross section of a further alternativeembodiment of a light source according to the present invention,

FIG. 6 shows schematically a cross section of a yet further alternativeembodiment of a light source according to the present invention,

FIG. 7 shows schematically a possible shape of a light source accordingto the present invention,

FIG. 8 shows schematically a longitudinal section of another alternativeembodiment of a light source according to the present invention,

FIG. 9 shows schematically the cathode and the anode as disclosed inFIG. 8,

FIG. 10 shows schematically a longitudinal section of yey anotheralternative embodiment of a light source according to the presentinvention, and

FIG. 11 shows schematically a longitudinal section of a furtheralternative embodiment of a light source according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown, in a schematic longitudinalsection, an embodiment of a light source according to the presentinvention, identified generally by the numeral 10, and especiallyintended for illumination purposes. It includes a container havingwalls, one of which is identified by the numeral 20. This wall 20 has anouter glass layer and is shown to be cylindrical. The cylinder 20 has anend 21, which is covered by an end cap 60. A sealing (not shown) isprovided between the end cap and the cylinder 20, in order to achieve anairtight sealing of the container. At the other end 22 of the cylinder20 an end cap 61 is provided, similar to the one arranged at the end 21,also provided with a sealing. Alternatively, at the end 22 there can bearranged a circular wall as a continuation of the cylinder wall 20, alsohaving an outer layer of glass. The container is sealed in order tomaintain the vacuum (approximately 10⁻⁶ torr) created when the containeris evacuated.

Inside the container and preferably coaxially therewith, a cathode 40 isarranged. This cathode is a cold cathode, especially a field emissioncathode. Its construction and function will be explained further below.

The light source is provided with electrical connections 51, 52, andmeans (not shown) for fastening of the cathode 40. The cathode 40 can besoldered to the caps 60, 61 or it can be adhered to the caps 60, 61 byan adhesive, preferably an electrically conducting adhesive. It couldalso be clamped to the caps 60, 61 by clamping means or gripped bygripping means. It is also possible that a circular wall, which is acontinuation of the cylinder wall 20, is provided with supporting,fastening or gripping means.

The cylindrical part 20 of the container walls surrounding the cathode40 consists of an outer glass layer 23, a phosphor layer 24 (acathodoluminescent phosphor) and an inner conductive layer 25 forming ananode. The phosphor layer is a luminescent layer, which upon electronbombardment emits visible light. The anode is preferably made of areflecting, electrically conductive material, e.g. aluminum. Byarranging an aluminum layer covering the phosphor layer, adverse effectson the vacuum by possible evaporation of the phosphor are avoided.

The electrical connection means 51, 52 connect the cathode 40 and theanode 25, respectively, to a feed and control circuit (not shown). Thoseconnection means preferably include conductive terminal pins whichextend through the cap 60 and are insulated from each other. Theelectrical connection means 52 could further include conductive fingersor similar, which are in contact with the anode layer 25 provided insidethe container. The openings for the electrical connection means 51, 52in the end cap 60 are airtight sealed. At the other end 22 of thecontainer wall 20, there can be arranged an end cap 61 similar to theend cap 60, to support the cathode 40. However, this end cap 61, at theother end 22, could be formed without electrical connection means.

The cathode 40 includes a relatively thin wire or rod, of electricallyconductive material, e.g. a nickel wire. The wire or rod preferably hasa circular cross section and its diameter is in the millimeter range,about one to a few mm, e.g. 0.5-5 mm or 1.5-2 mm. This provides for astrong and durable cathode, exhibiting a surface sufficient for a highemission of electrons. The area of the wire is also sufficient for thecurrent to be conducted there through.

FIG. 2 shows the light source of FIG. 1 in a cross section taken atII—II.

In operation, a DC voltage is supplied between the cathode 40 and theanode 25 by means of a feed and control circuit (not shown), which couldbe located in a housing, connected to the AC mains e.g. through anordinary lamp socket. The feed and control circuit supplies the voltagesto the conductive terminal connections 51-52, to which it is connected.Preferably connection 52 is at ground potential and connection 51 isnegative. When the voltage is applied, an electrical field is createdbetween the cathode 40 and the anode 25.

Due to the geometry of the light source according to the invention afavorable distribution of the electric field is obtained. The electricfield is strongest where a strong electric field is needed, forobtaining field emission, namely around the cathode. The followingformula gives the electric field strength in a structure according theinvention, having a central conductor coaxially surrounded by a circularcylindrical conductor:${{E(r)} = {\frac{V_{0}}{\ln\frac{R_{o}}{R_{t}}} \cdot \frac{1}{r}}},$where E(r) is the electric field strength at radius r with respect tothe central axis of the central conductor, V₀ is the voltage appliedbetween the conductors (cathode and anode in the light source), R_(o) isthe inner radius of the cylindrical conductor (the anode) and R_(i) isthe outer radius of the inner conductor (the cathode). In FIG. 3, whichschematically shows the cathode and the anode of FIG. 2, variables ofthe formula are indicated. As seen from the formula a very strongelectric field close to the cathode can be obtained with suitablyselected dimensions. Especially a small radius of the cathode (small r)will give a high electric field close to the cathode. The electric fieldlines will be concentrated around the cathode, and it can be seen as ifthe cathode were surrounded by a virtual extraction electrode.

In order to obtain field emission from the cathode, it is covered with afield emitting material, such as a layer of carbon nanotubes. Theelectric field is then further amplified around the field emitting tips,and an amplification factor (of the field) of 1000 and even more can beobtained. This can be seen as an amplification of the effect of saidvirtual extraction electrode. Taking this amplification factor (about1000) into account, the electric field needed to efficiently extractelectrons (by field emission) from a layer of nanotubes is about 1kV/mm. For further explanation and discussion of nanotubes it isreferred to the articles “Field emission from carbon nanotubes: acomparative study” by J M Bonard, J P Salvetat, T Stöckli, L Forró, AChâtelain, Proceedings of the 193^(rd) ECS sumposium, 1998, and “Fieldemission properties of multiwalled carbon nanotubes” by J M Bonard, FMaier, T Stöckli, A Châtelain, W A de Heer, J P Salvetat, L Forró,Ultramicroscopy 73 (1998) 7-15, which articles are incorporated hereinby reference.

The irregularities are formed by carbon nanotubes applied on the(cylindrical) surface of the wire or rod included in the cathode. Thenanotubes have a very short length, less than about 10 μm, and do notaffect the variable r in the formula since the diameter of the wire orrod of the cathode is selected in the mm range, about one to a few mm,e.g. 0.5-5 mm or 1.5-2 mm. The tips of the nanotubes have a radius ofcurvature being in the range 0.1-100 nanometers.

The applied carbon nanotubes can be of different types, e.g. single wallnanotubes or open or closed multi wall nanotubes. In this casecatalytically deposited multi wall nanotubes deposited in the form of afilm are suitable and can be applied by a simple process. Such nanotubesare suitable for depositing on a wire and they will be appropriatelyoriented by the process, with their respective longitudinal axis beingessentially perpendicular to the longitudinal axis of the wire. Further,application of nanotubes by a catalytic or alternatively CVD processresults in good uniformity and low manufacturing cost. Recent laboratorymeasurements confirm that the amplification factor is about 1000 incatalytically deposited nanotube films and that currents up to 10 mA/cm₂are obtained.

When the field strength is sufficient to cause field emission ofelectrons from the field emitting surfaces (tips) of the field emittingmaterial (nanotubes) of the cathode 40, the electrons will accelerateand travel towards the anode 25. Due to the high kinetic energy of theelectrons and the fact that the anode layer is relatively thin (lessthan 0.1 micron), the electrons will pass through the anode so as toenter the phosphor layer while still having sufficient kinetic energy toexcite the phosphor to luminescence, whereby visible light is emitted.The electrons will then return to the anode to be drained off. Theelectron bombardment will cause, besides light, heating of the cylinderwall 20. The glass layer will provide for the dissipation of the heat.The voltage is in the range of kV, typically about 4-8 kV. The voltagemuch depends on the type of phosphor used. New types of phosphor arecontinuously developed and because of that, the voltage must be adaptedto the specific type of phosphor used. Changing the type of phosphor andthereby the voltages will cause changes in the currents and the heatingof the cylinder wall.

If for example a phosphor layer 24 which needs to be bombarded withelectrons of about 8 kV in order to obtain a good efficiency, and thecathode 40 has a diameter of about 1 mm in order to assure that thenanotube layer has a sufficiently big surface to emit the current neededfor high light intensity, the above formula gives an electric field of 4kV/mm at the cathode surface with an inner diameter of the anode 25being 55 mm. With a cathode diameter of 1.5 mm, 3.7 kV/mm is obtained atthe cathode surface if the inner diameter of the anode 25 is 28 mm. Afield strength of about 4 kV/mm has been chosen in these examples to besafely above the 1 kV/mm needed.

For the example above with a cathode diameter of 1.5 mm and an innerdiameter of the anode being 28 mm, a length of 20 mm (anode and cathode)gives an electron emission surface of about 1 cm². From this surfaceelectrons corresponding to a current of 10 mA can be emitted. Thecorresponding phosphor surface is about 20 cm², which thus gives acurrent density of 0.5 mA/cm² at the phosphor surface. This is a toohigh density for continuous operation (for a high voltage of 8 kV, thiscorresponds to 80 W for a 20 mm long cylinder lamp).

With a light source according to the invention there is thus no problemto obtain currents, and consequently light intensities wellcorresponding to what is obtained from a classical fluorescent lighttube. As seen from the examples the outer diameter of a light sourceaccording to the invention can be made to correspond well to that of aclassical fluorescent light tube. As apparent from the description, thelight source according to the invention starts to emit lightimmediately, when a voltage is applied between the anode and thecathode.

Due to the geometry of the light source according to the invention, thedimensional tolerances are not required to be very exact, especially incomparison to light sources having a grid. This is apparent from theformula above, and contributes to low manufacturing costs.

FIG. 4 shows an alternative embodiment of a light source, according tothe invention, in cross section. What differs from FIG. 2 is thearrangement of the layers of the wall 20′. It includes an outer glasslayer 23′, which is covered, on at least a major part of its inside, byan electrically conductive transparent material forming the anode 25′.The anode 25′ then carries the phosphor layer 24′ on the inside. Theanode is made from e.g. ITO (indium tin oxide). To establish directelectrical contact with the anode 25′, conductive fingers can bearranged as mentioned above and some regions of the anode 25′ aretherefore not covered with phosphor. Alternatively, electricallyconductive surfaces being in contact with the anode can be applied on tothe phosphor layer. Those surfaces are small not to interfere with theoperation of the light source but of sufficient size to establishelectrical contact with the conductive fingers.

The operation of this embodiment illustrated in FIG. 4 is essentiallythe same as that of the embodiment illustrated in FIG. 2. However, afterleaving the cathode 40, the electrons will first hit the phosphor layerand excite it to luminescence, and thereafter they will be drained offby the anode. Since the electrons first hit the phosphor layer and donot have to pass through the anode layer before they hit the phosphorlayer, the voltage applied between the cathode and the anode can beabout 1-2 kV lower than in the embodiment illustrated in FIG. 2.

In the previous embodiments, the cathode 40 has been shown to bearranged concentrically with the container wall 20. However, it can benon-concentrically arranged as shown in FIG. 5. Here the center of thecathode 40 is located at a distance d from the center 26 of thecylindrical container wall 20. By this arrangement, the electric fieldwill be increased at portions of the container and decreased in otherportions. Hereby a possibility to control the light intensity isobtained, so that increased light intensities can be achieved in certaindirections. However, the electric filed around the cathode, theextraction field, will not be substantially changed due to thenon-concentricity for moderate distances d. If the inner diameter of thecylinder wall 20 is 20 mm and the outer diameter of the cathode is 2 mm,a distance d of 5 mm will cause higher current densities at the portionsof the cylinder wall closest to the cathode 40, but the electric fieldaround the cathode will still be sufficient for filed emission aroundthe cathode 40. For small distances d (e.g. around 0.1 mm) the effectsare almost none. This means that exact concentricity is not necessaryfor obtaining homogenous light emission.

In FIG. 6, a further embodiment of the invention is shown, where thecathode 40, i.e. the carrier (wire or rod) of the surface irregularities(the nanotubes), have a non-circular cross section. The cross sectionshown is elliptical, but could be any, having a smooth curve, i.e. notexhibiting any sharp corners. In this case the electric filed, thecurrent densities and the light intensities can be controlled in asimilar manner as in the previous embodiment of FIG. 5.

In earlier embodiments the container has been shown to be a straightcylinder. However other shapes are possible. In FIG. 7 a containerhaving the shape of a bent tube, is shown. The tube can be bent in acircular form or semi-circular, as shown.

Since nanotubes are conductive the core or carrier (the wire or rod) ofthe cathode 40 does not have to be conductive. It can be made of asemi-conductive or an isolating material. In such a case the nanotubesare applied in a continuous layer, and electrical connections areprovided to this layer. This is valid for all previous embodiments.

In the alternative arrangement disclosed in FIG. 8 the wall 20″ has anouter glass structure and is shown to be spherical. The sphere 20″ hasan end 21″ which is covered by an end cap 60″. A sealing (not shown) isprovided between the end cap and the sphere 20″, in order to achieve anairtight sealing of the container. Also here the container is sealed inorder to maintain the vacuum (approximately 10⁻⁶ torr) created when thecontainer is evacuated.

Inside the container and preferably centrically therewith, a cathode 40″is arranged. The cathode 40″ includes a relatively small sphere ofelectrically conductive, electrically semi-conductive or insulatingmaterial, e.g. of nickel. The radius thereof is in the millimeter range,about one to ten mm. This provides for a strong and durable cathode,exhibiting a surface sufficient for a high emission of electrons. Alsoin this case the cathode is a cold cathode, especially a filed emissioncathode. Its construction and function will be explained further below.

The light source is provided with electrical connections 51″, 52″, andmeans 70 for supporting of the cathode 40″. Said means 70 takes the formof a thin conducting rod 70 extending to the end cap 60″. The rod 70could be clamped to the cap 60″ by clamping means or gripped by grippingmeans.

The spherical part 20″ of the container walls surrounding the cathode40″ consists of an outer glass structure 23″, a phosphor layer 24″ (acathodoluminescent phosphor) and an inner conductive layer 25″ formingan anode. The phosphor layer is a luminescent layer, which upon electronbombardment emits visible light. The anode is preferably made of areflecting, electrically conductive material, e.g. aluminum. Byarranging an aluminum layer covering the phosphor layer, adverse effectson the vacuum by possible evaporation of the phosphor are avoided.

The electrical connection means 51″, 52″ connect the cathode 40″ and theanode 25″, respectively, to a feed circuit (not shown). Those connectionmeans preferably include conductive terminal pins which extend throughthe cap 60″ and are insulated from each other. The electrical connectionmeans 52″ could further include conductive fingers or similar, which arein contact with the anode layer 25″ provided inside the container. Theopenings for the electrical connection means 51″, 52″ in the end cap 60″are airtight sealed.

In order to specify an example the case is taken where the outsidesphere has a radius of about 30 mm, which is similar to a standardincandescent bulb: If the inner radius is chosen to be 2.5 mm, theelectric field strength at the surface of the inner sphere will be 3500V/mm for an applied voltage of 8000 volts. It is easy to draw a currentof say 5 mA from the nanotube layer on the surface of the inner sphere(0.8 cm²), which at the phosphor layer on the surface of the outersphere (110 cm²) will give a current density of about 45 microamps/cm².

The above calculations are given as examples for a perfect sphericalsymmetry. In reality one must of course take into consideration the factthat the central sphere is held in place by a thin conducting rod andthat the outer sphere has an extension to the socket (compare FIGS. 8,10 and 11).

Additionally, one can also consider cases where not the whole surface ofthe inner sphere is covered with phosphor, as is disclosed in theembodiment of FIG. 10.

Furthermore it is also possible to move the inner sphere to anon-central position in the outer sphere in order to increase the lightintensity in certain directions. This follows from the embodimentdisclosed in FIG. 11. According to an alternative arrangement, notdisclosed on the drawing, the glass sphere could be covered, on at leasta major part of its inside, by an electrically conductive transparentmaterial forming the anode. The anode then carries a phosphor layer onthe inside. The anode is made from e.g. indium-tin-oxide or indiumoxide. To establish direct electrical contact with the anode conductivefingers can be arranged as mentioned above and some regions of the anodeare therefore not covered with phosphor. Alternatively, electricallyconductive surfaces being in contact with the anode can be applied on tothe phosphor layer. Those surfaces are small not to interfere with theoperation of the light source but of sufficient size to establishelectrical contact with the conductive fingers.

In order to reduce existing disturbances of the electrical field withinthe container it may also be of advantage to encase the electricallyconducting rod holding the inner sphere, within a separate casing. Suchcasing then takes the form of a grounded metallic cylinder.

Finally it should also be noted that the lamp bulb and the central beadcould also be shaped differently from a sphere, e.g. like an ellipsoid,in order to influence the light distribution in different directions.

Although the invention is described by way of the above examples,naturally, a skilled person would appreciate that many other variationsthan those explicitly disclosed are possible within the scope of theclaims.

Although the embodiments include certain details for the electricalconnection and for the support of the different parts in the lightsource, it should be noted that they can be formed in many other ways,as should also be understood by a person skilled in the art, and thatthey do not limit the scope of invention.

1. A light source, comprising an evacuated container having acylindrical shape, a diameter in the range of about 8-80 mm, and walls,at least a portion of which comprises an outer glass layer which on atleast part thereof is coated on the inside with a layer of phosphorforming a luminescent layer, and a conductive layer forming an anode,which layer of phosphor is excited to luminescence by electronbombardment from a field emission cathode located in the interior of thecontainer, wherein the field emission cathode comprises an elongatewire-shaped carrier having a cylindrical surface and a firstlongitudinal axis, and at least a portion of said cylindrical surface isprovided with conductive surface irregularities in the form of carbonnanotubes, each having a second longitudinal axis being essentiallyperpendicular to the first longitudinal axis, and free ends of saidnanotubes forming tips having a radial extension less than about 10 μm.2. The light source according to claim 1, wherein the cylindricalsurface has a diameter in the range of 0.5-5 mm.
 3. The light sourceaccording to claim 1, wherein the elongate carrier is made of aconductive material.
 4. The light source according to claim 1, whereinthe elongate carrier is made of a semi-conductive material.
 5. The lightsource according to claim 1, wherein the elongate carrier is made of aninsulating material.
 6. The light source according to claim 1, whereinthe elongate carrier is coaxially arranged in the container.
 7. Thelight source according to claim 1, wherein the elongate carrier iseccentrically arranged in the container.
 8. The light source accordingto claim 1, wherein the elongate carrier has an essentially circularcross section.
 9. The light source according to claim 1, wherein theelongate carrier has a non-circular cross section with a smooth curve.10. The light source according to claim 1, wherein the elongate carriercomprises a wire.
 11. The light source according to claim 1, wherein theelongate carrier comprises a rod.
 12. The light source according toclaim 1, wherein the tips have a radius of curvature being in the range0.1-100 nanometers.
 13. The light source according to claim 12, whereinsaid nanotubes are arranged on the carrier in the form of a depositednanotube film.
 14. The light source according to claim 1, wherein thetips are essentially uniformly distributed around the carrier.
 15. Thelight source according to claim 1, wherein the luminescent layer isarranged between the glass layer and the anode, and the anode is made ofa reflective material for reflection of the light emitted from theluminescent layer.
 16. The light source according to claim 1, whereinthe anode is arranged between the glass layer and the luminescent layer,and the anode is made of a transparent material.
 17. The light sourceaccording to claim 1, wherein the phosphor layer is formed by aconductive phosphor and the phosphor layer also forms the anode.
 18. Thelight source according to claim 1, wherein the container has the shapeof a curved tube.
 19. A field emission cathode, for use in a lightsource, and to be at least partially encompassed by an anode having acylindrical shape and a diameter in the range of about 8-80 mm, andcomprising an elongate electrically conductive element, characterized inthat said elongate electrically conductive element has the form of acylindrical surface having a first longitudinal axis, and at least aportion of said cylindrical surface being provided with conductivesurface irregularities in the form of carbon nanotubes, each having asecond longitudinal axis being essentially perpendicular to the firstlongitudinal axis, free ends of said nanotubes forming tips having aradial extension less than about 10 μm.
 20. The field emission cathodeaccording to claim 19, wherein the elongate wire-shaped carrier is madeof a conductive material.
 21. The field emission cathode according toclaim 19, wherein the elongate wire-shaped carrier is made of asemi-conductive material.
 22. The field emission cathode according toclaim 19, wherein the elongate wire-shaped carrier is made of aninsulating material.
 23. The field emission cathode according to claim19, wherein the elongate carrier has an essentially circular crosssection.
 24. The field emission cathode according to claim 19, whereinthe elongate carrier has a non-circular cross section with a smoothcurve.
 25. The field emission cathode according to claim 19, wherein theelongate carrier comprises a wire.
 26. The field emission cathodeaccording to claim 19, wherein the elongate carrier comprises a rod. 27.The field emission cathode according to claim 19, wherein the tips havea radius of curvature being in the range 0.1-100 nanometers.
 28. Thefield emission cathode according to claim 19, wherein said nanotubes arearranged on the carrier in the form of a deposited nanotube film. 29.The field emission cathode according to claim 19, wherein the tips areessentially uniformly distributed around the carrier.
 30. A lightsource, comprising an evacuated container having walls, at least aportion of which comprises an outer glass structure which on at leastpart thereof is coated on the inside with a layer of phosphor forming aluminescent layer, and a conductive layer forming an anode, which layerof phosphor is excited to luminescence by electron bombardment from afield emission cathode located in the interior of the container, whereinthe field emission cathode comprises a carrier, at least partly takingthe form of a sphere, and at least a portion of the surface of saidsphere is provided with conductive surface irregularities in the form ofcarbon nanotubes, each having a longitudinal axis being essentiallyperpendicular to the surface of the carrier, the free ends of saidnanotubes forming tips having a radial extension less than about 10 μm.31. The light source according to claim 30, wherein said carrier is madeof a conductive material.
 32. The light source according to claim 30,wherein said carrier is made of a semi-conductive material.
 33. Thelight source according to claim 30, wherein said carrier is made of aninsulating material.
 34. The light source according to claim 30, whereinthe container at least partly takes the form of a sphere having a radiuswithin the range of 1-10 cm.
 35. The light source according to claim 30,wherein the carrier is arranged in the center of the container.
 36. Thelight source according to claim 30, wherein the carrier is eccentricallyarranged in the container.
 37. The light source according to claim 30,wherein the tips have a radius of curvature being in the range 0.1-100nanometers.
 38. The light source according to claim 30, wherein the tipsare essentially uniformly distributed on said portion and the surface ofsaid sphere being provided with surface irregularities.
 39. The lightsource according to claim 30, wherein the luminescent layer is arrangedbetween the glass structure and the anode, and the anode is made of areflective material for reflection of the light emitted from theluminescent layer.
 40. The light source according to claim 30, whereinthe anode is arranged between the glass structure and the luminescentlayer, and the anode is made of a transparent material.
 41. The lightsource according to claim 30, wherein the phosphor layer is formed by aconductive phosphor and the phosphor layer also forms the anode.
 42. Afield emission cathode, for use in a light source, and to be at leastpartially encompassed by an anode, and comprising conductive surfaceirregularities in the form of carbon nanotubes, each being provided onat least a portion of a carrier including a spherical surface and havinga longitudinal axis being essentially perpendicular to the surface ofthe carrier, and the free ends of said nanotubes forming tips having aradial extension less that about 10 μm.
 43. The field emission cathodeaccording to claim 42, wherein said carrier is made of a conductivematerial.
 44. The field emission cathode according to claim 42, whereinsaid carrier is made of a semi-conductive material.
 45. The fieldemission cathode according to claim 42, wherein said carrier is made ofan insulating material.
 46. The field emission cathode according toclaim 42, wherein the cathode is to be at least partially encompassed byan anode at least partly taking the form of a sphere having a radiuswithin the range of 1-10 cm.
 47. The field emission cathode according toclaim 42, wherein the tips have a radius of curvature being in the range0.1-100 nanometers.
 48. The field emission cathode according to claim42, wherein the tips are essentially uniformly distributed on saidportion and the surface of said sphere being provided with surfaceirregularities.
 49. The light source according to claim 18, wherein theshape of the curved tube is curved in a circular or semicircular curve.